An image sensor is a device including several million elements for transforming light into an electrical signal depending upon the intensity of radiation when it receives the light. The image sensor is installed in a digital input device, which enables recording of pre-digitalized images to digitalized images. Recently, the need for these devices has remarkably increased for use in various security devices and digital photos.
The image sensor is composed of a pixel array, which is a plurality of pixels arranged in a two-dimensional matrix format. Each pixel includes photosensitive means and transmission and signal output devices. Depending upon the transmission and signal output devices, the image sensor is broadly classified into two kinds: a charge coupled device (CCD) type of image sensor and a complementary metal oxide semiconductor (CMOS) type of image sensor.
In the image sensor, a color filter separates light coming from the outside into red, green, and blue and transmits the separated light to a corresponding photodiode in each pixel.
FIG. 1 is a schematic cross-sectional view of a structure of a conventional complementary metal oxide semiconductor (CMOS) image sensor. Hereinafter, the structure of conventional complementary metal oxide semiconductor (CMOS) image sensor 100 is described with reference to FIG. 1. The light coming from the outside reaches a color filter 150 through a microlens 110 and a second overcoating layer 130, is separated at the color filter 150 into each of red, green, and blue (R, G, and B), and is then transmitted to a photodiode 113 corresponding to each pixel formed between metal layers 180 on an interlayer dielectric layer 115 through a first overcoating layer 170 and a passivation layer 190. A pad 160 is formed to connect to the photodiode 113 in order to draw the signal from the image sensor 100.
Much research directed to new processes and related materials has been undertaken to produce an image sensor. In particular, research is actively focusing on improvements in display quality of minute pixels with sizes decreasing from 3 to 5 μm to 1 μm. Due to the small pixel size of 1 μm or less, a geometric optical aspect and a wave optical aspect toward the microlens have to be considered at a level of 1.5 times the visible ray wave length.
Further, as the unit pixel becomes smaller, the diameter of the microlens becomes smaller. As a result, the crosstalk phenomenon with an adjacent pixel more easily occurs unless the focal distance of the lens is decreased, which can deteriorate resolution. In order to solve this problem, the thickness of each layer formed between the photodiode and the microlens can be decreased. In addition, Korean Patent Laid-Open Publication No. 2007-0033748 is directed to a production method which does not form an upper overcoating layer.
Other image sensors and methods for making the same are disclosed in many patents, such as Korean Patent Laid-Open Publication No. 2002-039125, Japanese Patent Laid-Open Publication No. Pyong 10-066094, Korean Patent Laid-Open Publication No. 1998-056215, Japanese Patent Laid-Open Publication No. Pyong 7-235655, Korean Patent Laid-Open Publication No. 2003-056596, Japanese Patent Laid-Open Publication No. 2005-278213, Korean Patent Laid-Open Publication No. 2003-002899, and Japanese Patent Laid-Open Publication No. Pyong 11-354763.
Korean Patent Laid-Open Publication No. 2006-0052171 provides a fine pattern of 2.0×2.0 μm using a dye in order to obtain a high density pixel, as in Japanese Patent Laid-Open Publication No. 2004-341121. However, a dye is used as a colorant instead of a pigment in this method. The dye has problems relating to long term reliability because the photoresistance or thermal resistance and other properties thereof can deteriorate.
In addition, Japanese Patent Laid-Open Publication No. P7-172032 is directed to the use of a black matrix in order to prevent the colors of red, green, and blue from mixing and dislocating. However, this method also requires an additional process; it is almost impossible to make a fine black matrix; and in addition, the aperture ratio is decreased by introducing the black matrix.